How Does Precipitation Form in a Cloud?
The “rain-making” processes in a typical cloud are fascinating and mysterious to meteorologists. As I child, I often wondered, “Why don’t the clouds fall to the ground like the rain drops?” After all, clouds are simply composed of tiny liquid cloud drops, ice crystals or both. Yet, the processes that allow clouds to produce rainfall and other forms of precipitation are far from simple. Precipitation that falls from clouds usually results from large water drops, ice crystals that have melted, or ice particles that stay frozen (e.g. snow or sleet). To understand precipitation processes, we must begin with basic physics. Clouds are created by air that rises until it cools to the dew point. At this point, the air is saturated and vapor molecules seek to condense to form small water droplets. However, in the atmosphere, condensation from vapor to gas is not a straightforward process. In the atmosphere, water vapor molecules need small particles called condensation nuclei (CN) to cling to. C
